(a) Field of Invention
The present invention relates to rotary piston engines, and, more particularly, to rotary piston engines having means for expelling combustion gas from working chambers.
(b) Prior Art
Conventional Wankel type of rotary piston engines comprises a casing which includes a rotor housing having an inner wall of trochoidal configuration and a pair of side housings gas-tightly secured to the opposite sides of the rotor housing to define a rotor cavity of trochoidal configuration. A substantially polygonal rotor is disposed in the rotor cavity for rotation with its apex portions in sliding contact with the inner wall of the rotor housing to define working chambers between the inner wall of the rotor housing and flanks of the rotor. Each working chamber is, therefore, displaced along the inner wall of the rotor housing as the rotor rotates and has a volume which varies in dependence upon the rotation of the rotor through intake, compression, expansion and exhaust strokes. The casing is formed with one or more intake ports located to open to the working chamber which is in the intake stroke so that air or air-fuel mixture may be introduced therein. The casing is also formed with an exhaust port located to open to the working chamber which is in exhaust stroke.
In this type of rotary piston engine, it has been experienced that a certain amount of combustion gas is carried over into the working chamber resulting in a dilution of air-fuel mixture. Such dilution of air-fuel mixture often causes misfiring and possibly results in rough engine operation. Such carry-over of combustion gas is mainly caused by the fact that the rotor drives residual gas toward the intake area of the engine. Referring to one of the working chambers, a certain amount of combustion gas remains therein at the end of the exhaust stroke because the combustion gas is not completely exhausted through the exhaust port. Such residual combustion gas is carried by the rotating rotor to a position where the intake stroke takes place and is mixed with the intake air or air-fuel mixture introduced through the intake port into the intake working chamber.
Combustion gas is additionally introduced into the intake working chamber by overflow from the exhaust port. As well known in the art, in this type of rotary piston engine, there is a so-called overlap period wherein a working chamber simultaneously communicates with both of the intake and exhaust ports. It has widely been recognized that in this overlap period the combustion gas in the exhaust port is allowed to flow through the exhaust port into the intake working chamber.
The absolute amount of combustion gas thus carried over into the intake working chamber is thought to be significantly unaffected no matter how the charging efficiency of the engine is. Therefore, the carry-over ratio, that is, the ratio of the amount of such carried over combustion gas to the amount of total intake gas, is increased substantially proportionally to a decrease in the charging efficiency. On the other hand, the allowable limit of the carry-over ratio, which is defined as the maximum carry-over ratio where the number of misfires can be suppressed under 10 times per minute, will be decreased in response to a decrease of charging efficiency. Furthermore, the allowable limit of the carry-over ratio has a tendency to abruptly decrease in a lower range of the charging efficiency.
Thus, it has been recognized that there is a threshold where the actual carry-over ratio exceeds the allowable limit at a certain value of the charging efficiency. Under such conditions, beyond the allowable limit, there will be a significant increase in the possibility of misfire and in some adverse circumstances the engine may fail to operate. In order to prevent the above-mentioned problems, it has been required to maintain the charging efficiency at an adequately large value so that the actual carry-over ratio is always smaller than the allowable limit.
For the sake of recent improvements of internal combustion engines in respect of engine efficiencies, such as gas-tightness and frictional resistances, the requirements on the charging efficiency for maintaining idling operation have been moderated. However, the charging efficiency in idling operation has still been maintained at a relatively high value for the purpose of preventing misfire.
One example concerning this problem is disclosed in Japanese Pat. No. 53-39926 (corresponding to U.S. Pat. No. 4,116,190). This patent teaches that the casing should be provided with a take-out port located to open the compression working chamber for drawing compressed air from the compressing working chamber and a discharge port located between an intake port and an exhaust port to the exhaust working chamber, and that the take-out port should be connected through a throttle-controlled valve with the discharge port. However, the discharge port is not able to be widened enough because a large discharge port allows large amounts of combustion gas to flow from the exhaust working chamber to the intake working chamber while apexes of the rotor pass the discharge port.
Another example is disclosed in Laid-Open Japanese Utility Model Application No. 60-0149836. This application teaches that a port insert being inserted into the exhaust port should have the air discharge passage at the leading end of the port insert. The air discharged from the air discharge passage forms an air curtain separating the working chamber just changing from exhaust stroke to intake stroke into the leading side and the trailing side, and prevents combustion gas from being carried over. However, since the air discharge passage is merely located at the leading side of the exhaust port, most air is exhausted with the combustion gas, and the air curtain is destroyed if the amount of air used is small. Thus, the air curtain needs a great amount of air, and large power needs to be expended to supply the air from the air pump thereby consuming much energy.